1. Technical Field
The present invention relates to electrochemical methods and systems for calculating corrosion rate, particularly to methods and techniques for evaluating both generalized and localized corrosion, and most particularly relates in a non-limiting embodiment, to methods and techniques for simultaneously determining and calculating generalized corrosion and localized corrosion in hydrocarbon pipelines, transportation systems, processing vessels and fluid handling equipment.
2. Description of the Related Art
General corrosion rate measurements, such as Linear Polarization Resistance (LPR) measurements, typically involve some sort of external polarization, where the resultant current response is used to calculate the Resistance to Polarization, Rp. These methods do not readily give a measure of the localized corrosion.
Localized corrosion of equipment is a serious problem in many industries and processes. In particular, corrosion failures in many oil and gas production systems, oil/gas/water transmission pipelines, petrochemical and chemical processing plants, fossil fuel and nuclear power plants involve localized corrosion. Localized corrosion may result in loss of production, increase in maintenance cost, environmental pollution and potential health and safety hazards, etc. It is important that the occurrence of localized corrosion is identified and the severity determined in advance of structural failure, particularly catastrophic failure. In addition, the ability of chemical introduction and other techniques to inhibit and/or prevent localized corrosion needs to be determined.
Localized corrosion is the selective removal of metal by corrosion at small areas or zones on a metal surface in contact with a corrosive environment, usually a liquid. While pitting is a localized corrosion, the locally corrosive pits may eventually cover substantial portions of a corroded electrically conductive article's surface. Localized corrosion may occur when small local sites are attacked at a much higher rate than the rest of the surface. Alternatively, a film or surface may protect the majority of the structure, while a relatively small area is under localized corrosion attack. Localized corrosion occurs when corrosion works with other destructive forces such as stress, fatigue, erosion and chemical attacks. Localized corrosion can cause more damage than any of these destructive forces individually.
The problems resulting from localized corrosion have been dealt with for many years with variable success. Localized corrosion is highly stochastic in nature and its occurrence is fairly unpredictable. Currently, localized corrosion is studied or monitored by measuring directly relatively large features (e.g. pits) on the surface by using standard optical microscopy with limited spatial resolution. Indirect methods are also used, such as electrochemical noise, to provide indication of the probability of localized (e.g. localization index) corrosion.
Although the above techniques are widely employed, they (1) possess limitations in that they only provide information on uniform (general) corrosion conditions because they provide an average signal for the surface of the electrode being monitored; and (2) depending upon the environment, metallic material, and corrosion type, the assumption that the corrosion rate is inversely proportional to the measured charge transfer or polarization resistance is invalid because the corrosion is of a localized nature. These problems have been addressed by monitoring localized corrosion via the utilization of electrochemical potential noise analysis.
Electrochemical noise (ECN) may be defined as the spontaneous fluctuations of current and potential generated by corrosion reactions. Various methods have been used to determine corrosion rates, including LPR. In LPR a direct current (DC) signal is applied to a corroding cell consisting of two or three electrodes and the resulting DC polarization is monitored. Provided that the applied current is small and that the potential shift is less than 20 millivolts (mV), the response is linear in most cases and the measured resistance, commonly known as the Polarization Resistance, Rp, may be related inversely to the rate of the uniform corrosion attack. Other techniques include the application of electrochemical impedance spectroscopy (EIS) in which a sine wave current or potential is applied. In a similar manner to the linear polarization technique, the sine wave potential or current resulting from the applied current or potential is monitored. Alternatively, a pseudo random noise signal can be applied to a corroding cell, with the electrochemical impedance obtained by time or frequency domain transformations.
The ECN technique tries to correlate the two signals using a range of mathematical methods. Standard mathematical techniques however simply treat the data as noise and work on factors such as the standard deviation and the mean. General corrosion is obtained from the ratio of standard deviation of voltage and standard deviation of current (Rn=σV/σI), while localized corrosion is defined as a ratio of standard deviation of current and mean value of current (LI=σI/rmsI). Localized corrosion can produce significant individual events that are analyzed incorrectly if they are considered as noise.
Alternatively, by coupling current analysis with electrochemical potential noise analysis further information can be obtained. For example, two similar electrodes can be coupled together via a zero-resistance ammeter (ZRA) with the output of the ZRA passed to the input of the electrochemical noise analysis system. In this way, the fluctuation of the coupling current may be analyzed in essentially a similar manner as for the electrochemical potential noise analysis described previously.
Systems which employ two working electrodes fabricated with the same material and exposed to the same corrosion conditions as the metallic surface to be tested are known. Such systems further employ a device for measuring the coupling current between the working electrodes, a device for measuring electrochemical potential noise originating from the electrodes, and a device for comparing the coupling current with the electrochemical current noise to provide an output indicative of the degree to which corrosion is localized. The systems utilize open circuit potential conditions, employing two working electrodes in an electrolyte environment wherein both electrodes are short circuited with a low resistance amp meter. The current between these two working electrodes is the result of corrosion occurring on them, with the measurement of the net current relating to the corrosion on both of them. Disadvantages of this system, however, range from the fact that the working electrodes need to be identical to obtain accurate readings and obtaining such identical electrodes is difficult, if not impossible. Another problem is that it is unknown which electrode is responding to reveal the corrosion, due to the fact that this system requires the use of two working electrodes which limits where such systems can be employed. Furthermore, distinguishing between various types of localized corrosion is, at minimal, difficult due to the fact that both electrodes contribute to the system response.
Localized Corrosion Monitoring (LCM™) looks at the potential of an electrode with respect to time. Individual localized events tend to give a potential transient typically due to a pitting, or any other form of localized corrosion (e.g. crevice, SCC), event. The magnitude of the current involved in this pitting event can be calibrated with a periodic polarization using either a small LPR type polarization, or a larger scale polarization of the same magnitude as the pitting event. In this way a V/I conversion curve can be constructed for conversion of the potential transient into a current transient. The current can then be used to calculate charge and ultimately metal loss associated with the individual pitting event.
What is needed in the art is a simplified corrosion rate detection system and method by providing corrosion detection calculation capability for both generalized and localized metal corrosion, without the use of external polarization required for LPR and LCM techniques.